"I believe the o/d unit of the A500 and A518 and variants could be adapted "

I don't understand...adapted to what?

Suggesting adapting the overdrive unit to the 904. This was essentially what Chrysler did when it made the A500. However, you cannot adapt the overdrive unit to an 904. The A500 has additional passages drilled in the body taht cannot be drilled into the 904 case.

I agree. But the factory did this for you....the 904 with an OD grafted on is an A500.

If it was me, and I wanted a 904 OD (A500) for a Slant, I'd cut the bell off a 904 and weld it on to the small block A500.

I'm not a huge fan of the OD used by Chrysler. It's strong enough, but way more clunky than it should be. I also suspect it wastes more power than it should.

That's true, but the adapter has holes where you want it to have holes. I'd have to make the adapter. I'm mostly thinking of manual transmissions because I want it to shift manually and to use the regular foot clutch. But it could be used with an automatic just as the Gear Vendors does, but that's where it can get clunky. You can setup a Laycock remotely so that it uses a short drive shaft to the transmission output shaft. I don't think there is anything in the planetary gear set that is inherently inefficient, but I'm definitely not the expert! The popular ZF 8-speed uses 4 separate planetary gear sets.

I think it would need a pump because the oil would get hot and damage the o/d unit. That's where this idea becomes to complex. I don't know of any pump to use. A pump that was concentric with the adapter shaft might be best. Electric pumps probably wouldn't last very long. Its a crazy idea, but when the Laycock was first used in the 1930s, I don't think they expected to see them still being built 80 years later and sell for $3,100 as a kit.

The Laycock and the Gear Vendors o/d use a separate oil reservoir. Many forget to have that serviced and this leads to early wear in the o/d unit. The OEM o/d from GM and Chrysler, of course use the same oil supply for the o/d. I think the inefficiencies with the o/d are in the classic Simpson Cycle design and the torque converter. The Simpson Cycle patent was received in 1949, its a great design. I think the Laycock might have similarities to the Ford Model T planetary transmission. The Borg Warner o/d was in use to about 1970 as an OEM product, then it disappeared.

The pump I use on the clutchflite would work.

I adapted and ran an A500 in my 68 Dart for 15 yrs. It was quite a bit of hacking and welding to get it to fit, and I'm not sure I ever got the DS angles right. It worked fine, but it sapped a bunch of power (strip testing before and after), and it was about 80lbs heavier than the 904, if you include the adapter plate. I would not suggest anyone do that swap unless you have a free/cheap A500 and plate, OR you have a truck like Reed and you are not going for efficiency.

The car has a hopped up 200-4R in it now and it gets about 10% better MPG and I dropped at least 0.3 sec in the 1/4 mile w/o changing anything else. I bit of that is converter stall. I could never get a lockup converter made for the A500 that would stall above 2400 and the new one is around 2600. The 200-4R with adapter is 40-50 lbs lighter than the A500.

Lou

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Interesting. Efficiency is actually why I am swapping in the overdrive. The truck was built with a 727 and 3.9 rear gears and it is a total pig off the line. I can floor it and it still accelerates slowly. Vacuum advance works, mechanical timing advance works, carb is good, the truck just takes forever to accelerate. It will eventually get up to 80 miles per hour on the freeway, but it takes some time to get there. I am hoping that switching from a 727 to an A500 will (a) save a little weight (although now I don't know if it will) (b) bring highway RPMs down and (c) lessen the parasitic power loss through the trans. I wish I had better means of tracking parasitic power loss, fuel economy, and other parameters, but I will have to make do with keeping an accurate fuel mileage log. I need to get some aftermarket gauges installed (vacuum, O2 sensor, etc...), verify the acuracy of my fuel gauge and odometer, and take some careful notes before I install the A500.

As far as installation goes, I am lucky in that I grabbed the trans crossmember from a Dakota and a trans crossmember from a 90 D series pickup. The 90 trans crossmember was the same for the three and four speed auto transmissions. When I install the A500 in my truck I am going to use a 360 block as an alignment tool to get all the installation angles and placement correct. I will remove the slant and 727 completely and temporarily install the 360/A500 combination in order to determine where the trans rear support crossmember should be installed. I'm hoping I can use the factory 1990 trans support since the frame on the 76 D100 should be very similar to the frame on the 1990 D150. Once I get the transmission placed correctly, I am going to modify a spare set of engine mounts to allow the slant to sit the same distance closer to the radiator as the thickness of the adapter plate. I will almost certainly have to run an electric fan to accomplish this, but it seems the easiest way to make room for the adapter.

I'm probably missing something but I'd leave the engine in the stock position and let the trans move back to accommodate the adapter. Especially in a truck where you have the most room. Moving the engine forward disturbs all sorts of stuff.

The problem with the internet is everyone has a different idea of what a 'pig' runs like. Your truck might be perfectly fine and you just think it should be faster. But my guess is it is a pig and is not running as well as it should. I'd fix that first. I mean, 80MPH might be the top speed (or close to it) but it shouldn't take forever to get there.

This is all good advice that I agree with. However, every single other slant six vehicle I have owned or driven has accelerated quicker than this truck, and that includes vans with 3.2 rear gears. Something is wrong somewhere in the drivetrain. Maybe a failed torque converter, maybe sticking weights in the distributor, I just haven't spent the time to diagnose it.

I want to leave my options for engine choice as open as possible in case I decide to get that 360 running and swap it in. Because of that, I am trying to install the A500 in a location where it will not have to move if I run a slant with an adapter or a smallblock without an adapter. The easiest way I can think of to do that is to elongate the holes in the slant motor mounts.

I think the A-500 and A-518 might have worked better if the o/d ratio were reduced. With EFI and better engine management maybe the motors today can better accommodate o/d. I think think the GM 700R4 was very good either. The 200R4 has a better design, I've read. The 200R4 efficiency goes against my "engine management" theory because it has 0.67:1 final drive and was last used in '87, must be close to lugging the motor in many scenarios. Does anyone know the architecture of the 200R4,has anyone been inside one? Do they mostly use less power because the internals are lighter?

The OD ratio of the A500 is very close to that of the 200.

Yes, the 200 and many other transmissions are more efficient. Without going into all the 'politics' of GM versus Mopar, it's probably best explained by the fact the 200 was designed many decades after the 904 and at a time when efficiency was more heavily weighted. It's a smaller and lighter trans, too.

GM stopped building the 200R4 in 1990, but has since become popular with project builders. It doesn't require any digital computer.

Greg has got it. The A500 was a band aid on the already built 904 and 727 platforms and was designed for trucks, where most folks don't care about MPG. Somewhere in there, you are losing power. The 80s were much better for efficiency designs.

Lou

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